CODEWITHSUNDEEP
c++c++17language-lawyer
xmh0511
xmh0511

Reputation: 7369

Is it a defect in basic.def.odr section

struct S { static const int x = 0; };
int main(){
  S obj;
  int v = obj.x;
}

Consider the above code, Is obj.x an odr-use? According to look at the section of basic.def.odr#3

A variable x whose name appears as a potentially-evaluated expression ex is odr-used by ex unless applying the lvalue-to-rvalue conversion to x yields a constant expression that does not invoke any non-trivial functions and, if x is an object, ex is an element of the set of potential results of an expression e, where either the lvalue-to-rvalue conversion is applied to e, or e is a discarded-value expression.

We have such analysis as the following:

In my code, the variable name is x, which is the x in the above quote, and the expression obj.x is the ex in the quote. So, Is obj.x a potentially-evaluated expression? Yes, It is, because of this:

An expression is potentially evaluated unless it is an unevaluated operand or a subexpression thereof.

obj.x is neither an unevaluated operand or a subexpression thereof. So obj.x is a potentially-evaluated expression. And applying the lvalue-to-rvalue x indeed yields a constant expression because it's initialized by 0. What I doubt is the following, that is,

and, if x is an object, ex is an element of the set of potential results of an expression e

we assume the e is obj.x because lvalue-to-rvalue conversion will apply to it. And what's the set of potential results of e? As following:
basic.def.odr#2

The set of potential results of an expression e is defined as follows:

  • If e is a class member access expression, the set contains the potential results of the object expression.

Because e is a class member access expression. hence its set of potential results is object expression.

expr.ref#3

Abbreviating postfix-expression.id-expression as E1.E2, E1 is called the object expression.

So, the object expression here is obj, hence the set of potential results of expression obj.x contains obj. Obviously, here ex is xand if e isobj.xthen its set of potential results of expression would beobj. So, what the expression eof which the exx` is a element of set of potential results? According to look at basic.def.odr#2, I find nothing.

I only pretty sure the lvalue-to-rvalue conversion is applied to obj.x whole expression. However, all compiler all agree the use of obj.x is none odr-use. Is it a defect in the standard?

Upvotes: 0

Views: 121

Answers (1)

Davis Herring
Davis Herring

Reputation: 39878

You can’t rely on a compiler to tell you whether it’s an odr-use; if it is, and you haven’t defined the variable, the program is ill-formed, no diagnostic required. That said, it seems like this was a defect in C++17 in that the possibility of referring to a static member variable with a class member access (as opposed to a qualified-id) was overlooked.

Upvotes: 2

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